Aluminum and glass are now mainstream materials as substrate materials for information recording media such as a hard disk, and the like. As a glass substrate, a chemically strengthened glass is used as described in Japanese Patent No. 3412804, and an information recording medium using such a chemically strengthened glass is imparted with full reliability.
Meanwhile, in recent years, in the production of an information recording medium such as the above hard disk, it has come to be necessary to heat-treat a substrate at a high temperature for imparting a film formed on the substrate with novel properties, for example, for obtaining a film structure of a perpendicular magnetic recording mode. Aluminum as a typical substrate material has a problem that when exposed to a high temperature, an aluminum substrate is deformed and impaired in flatness that a hard disk is required to have, so that aluminum is not suitable for the above use. On the other hand, a glass substrate is also required to have high heat durability (high glass transition temperature) so that it can maintain sufficient flatness even when treated at high temperatures.
However, even if a glass constituting a glass substrate is improved in heat durability alone, the glass substrate may break due to a thermal shock when exposed to a sharp change in temperature.
That is, even a glass substrate formed of a glass having a high glass transition temperature sometimes undergoes a breaking when rapidly cooled close to room temperature from a state where it is heated at a high temperature.
As a glass substrate, further, there is demanded a highly reliable substrate that has high strength against scratching and that does not break in a production step.